The Oxone-mediated syn-dihydroxylation is a significant step in the area of metal-free dihydroxylations in general and for the synthesis of the indene acetonides in particular. Previous reports on the synthesis of these compounds are multistep in nature and employing explosive reagents such as peroxides for epoxidation, and strong acid treatment for the epoxide opening and then protection of the resulting diol as acetonide. Regioselectivity during the epoxide opening is one of the biggest problems till date.
The 1,2-diol unit is one of the most ubiquitous moiety in the natural products, pharmaceuticals and arguably has attracted significant amount of synthetic maneuvers since the beginning. The syn-/anti-dihydroxylation of alkenes has been considered as a simple approach for the installation of these 1,2-diol units, which in general has been mainly reserved for the metal-based reagents. The OsO4, introduced by Criegee, is the most commonly employed catalyst for the syn-dihydroxylation of olefins and was central to many developments in this area. The toxicity of osmium taken together with its volatility has led to explore the several other metals for this purpose; however, in general the metal-based syn-dihydroxylations have limitations in terms of the waste generated and also because of the difficulties in complete removal of the metal impurities from the products. Thus the metal free method for the dihydroxylation of alkenes is the most desired alternative and renewed interest being shown in the recent years. The phthaloyl peroxide (PPO), is the first reagent that has been employed for the metal free direct dihydroxylations by Greene. Though the reaction is stereospecific and documented as early as in 1956, the problems associated with instability/explosive nature of the reagent and the poor product yields have been dealt only recently by introducing cyclic acyl peroxides by Tomkinson and co workers. The closely related more reactive cyclic peroxides, the dioxiranes, have been widely employed for the epoxidations. The one-pot combination of olefin epoxidation and subsequent opening leading to vicinal diols has been also well established with the dioxiranes, however, it results either in trans-dihydroxylation products or diastereomeric mixture depending upon the conditions. The direct dihydroxylations with dioxiranes are rare. In 2000, White and co-workers, later in 2008 De Kimpe and co-workers reported the syn-dihydroxylation as a side reaction during the synthesis of patulin and mollugin. The corresponding acetonides have been isolated along with the expected epoxides. In both the instances, the scope of the reaction has been limited to specific substrates. These are unexpected and important findings and yet the potential of this syn-dihydroxylation with Oxone-acetone combination has been not yet tapped.